1. Field of the Invention
This invention relates to the field of voltage and current clamps, and particularly to clamping schemes to prevent feedback amplifiers from being overdriven.
2. Description of the Related Art
There are many electronic circuit applications in which it is necessary to prevent the voltage at a circuit node or the current through a circuit component from exceeding a particular value, i.e., to "clamp" the voltage or current at a certain level. Many clamp circuits exist to perform this function.
The need for a clamp circuit often arises when designing an amplifier, to prevent the amplifier from being "overdriven". When overdriven by an excessive input voltage, an amplifier's output no longer varies linearly with changes at its input, which typically happens when the amplifier's output stage becomes saturated. A voltage clamp can be connected to limit the voltage presented at either the input to an amplifier or to the amplifier's output stage, to prevent the amplifier from being overdriven.
When an amplifier is operating in its linear region, a clamp circuit connected to a circuit node ideally will not introduce noise or distortion into the signal at that node. Previous clamp designs have fallen short of this ideal. A simple clamp design is shown in FIG. 1, comprising zener diodes Z1 and Z2 connected to the input of an amplifier A1, to respectively limit the maximum positive and negative voltages that can be presented to A1. Unfortunately, this clamp scheme often affects the performance of the amplifier due to parasitic effects. Since the clamp is connected to the input of the amplifier, it may also have an adverse effect on the circuit driving the input. In addition, the clamp must dissipate large amounts of current when one of the zener diodes is conducting.
Another clamp scheme is depicted in FIG. 2, shown connected to a circuit node 10 at the interconnection of the input stage 12 and output stage 14 of a Class A feedback amplifier. At this location the clamp does not directly affect the circuitry driving or being driven by the amplifier. The emitters of an npn transistor 16 and a pnp transistor 18 are both connected to the circuit node. The bases of the npn and pnp devices are connected respectively to negative and positive reference voltages V.sub.REFN and V.sub.REFP. The base-emitter junction of the npn transistor 16 becomes forward-biased when the node drops to a voltage equal to V.sub.REFN -V.sub.be, where V.sub.be is the base-emitter threshold voltage for conduction, typically about 0.7 volts. This turns on the transistor 16 and thereby prevents the node voltage from going any more negative, thus clamping the voltage at V.sub.REFN -V.sub.be. The pnp transistor 18 operates similarly, clamping the node at a maximum positive voltage of V.sub.REFP +.vertline.V.sub.be .vertline..
There is an inherent problem with the clamp circuit of FIG. 2, however. Each transistor 16, 18 has a respective parasitic capacitance C.sub.je1, C.sub.je2 across its base-emitter junction. This capacitance varies non-linearly with the base-emitter voltage, as follows: ##EQU1## where C.sub.j0 is the zero-biased base-emitter depletion capacitance, V.sub.BI is the built-in potential of the base-emitter junction and m=1/2 for uniform doping and 1/3 for linear doping. A varying voltage at the circuit node 10 modulates each transistor's base-emitter voltage, which results in a C.sub.je1 and C.sub.je2 that vary non-linearly with the voltage at the circuit node 10. This non-linear capacitive effect is a source of distortion that degrades the amplifier's performance. The adverse effects of C.sub.je are made worse when the clamp is connected to a high impedance node as is typically found between the input and output stages of an amplifier, due to the longer RC time constant that results from the high impedance in combination with C.sub.je. C.sub.je and its effects are discussed, for example, in G. Neudeck, The Bipolar Junction Transistor, Volume III, Addison-Wesley Publishing Co., (1989), pp. 80-81.
Other problems can arise when using a clamp circuit to prevent the signal presented to the output stage of a feedback amplifier from exceeding certain limits. FIG. 3 shows a known Class-AB feedback amplifier having an input stage 20 that is connected to an output stage 22 at a high impedance circuit node 24. The input stage includes transconductance amplifiers 26 and 28, respective high impedance voltage inputs 30 and 32, low impedance current inputs 34 and 36, and push-pull current outputs 38 and 40. A resistor R.sub.gm interconnects the two current inputs 34 and 36. The input voltage V.sub.in is connected to the voltage input 30 of one transconductance amplifier 26. The output stage 22 includes an output amplifier 41 that produces an output voltage V.sub.out which is fed back via a voltage divider comprised of resistors R1 and R2 to the voltage input 32 of transconductance amplifier 28. An amplifier of this type is disclosed in U.S. Pat. No. 5,410,274 to Birdsall et al. A clamp circuit 42 is connected to the node 24 to prevent positive and negative excursions of the signal feeding the output stage 22 from exceeding positive and negative clamp voltage limits, designated V.sub.CLAMP+ and V.sub.CLAMP-, respectively, to prevent the output stage from being overdriven.
When operating in its linear region, the amplifier's output voltage V.sub.out is equal to V.sub.in .times.(1+(R1/R2)). Due to the action of the feedback loop, the voltage at voltage input 32 is about equal to V.sub.in, making the voltage across R.sub.gm small. A problem arises when the signal at node 24 reaches either V.sub.CLAMP+ or V.sub.CLAMP-, so that the clamp circuit 42 prevents the voltage fed to the output stage 22 from increasing or decreasing further. This breaks the amplifier's feedback loop and causes the voltage at voltage input 32 to be held at the clamp voltage multiplied by R2/(R1+R2). Any further increase in V.sub.in causes the voltage drop across R.sub.gm to increase, thereby generating a current through R.sub.gm which can become significantly large since R.sub.gm is typically small in a Class-AB amplifier. R.sub.gm can become damaged if the current through it becomes too large, as can components connected to R.sub.gm. Therefore, a need exists for a clamping circuit that will limit the current through a resistor interconnecting two transconductance amplifiers in a Class-AB feedback amplifier.